1. Field of the Invention
This invention relates to a process for the preparation of triphenylamine compounds. More particularly, it relates to a process for the preparation of triphenylamine compounds which are useful as charge transport materials (or hole transport materials) contained in electrophotographic sensitive materials.
2. Description of the Prior Art
Conventionally, inorganic photoconductors such as selenium (Se), cadmium sulfide (CdS), zinc sulfide (ZnS) and amorphous silicon (a-Si) have been known to be useful in photosensitive materials for use in electrophotography. Although these inorganic photosensitive materials have many advantages, they also have various disadvantages, for example, in that they are harmful, they cannot be easily disposed of, and they are expensive. For this reason, a variety of organic photosensitive materials using organic substances free of such disadvantages have recently proposed and put to practical use. From the viewpoint of structure, such photosensitive materials include separated-function type photosensitive materials in which a material for generating charge carriers (hereinafter referred to as the "charge-generating material") and a material for accepting the generated charge carriers and transporting them (hereinafter referred to as the "charge transport material") are formed into separate layers, and single-layer type photosensitive materials in which charge generation and charge transport are effect in the same layer. However, separated-function type photosensitive materials are more commonly used because they have a wide choice of materials and they can be designed to have high sensitivity.
As the charge transport medium, there may be used either a polymeric photoconductive compound such as polyvinyl carbazole, or a low-molecular-weight photoconductive compound dispersed or dissolved in a binder polymer. Polymeric photoconductive compounds, when used alone, have insufficient film-forming properties and bonding properties. In order to improve these properties, a plasticizer, a binder polymer and the like are added thereto. However, these additives may cause a reduction in sensitivity and an increase in residual potential, so that the resulting products cannot be easily used for practical purposes. On the other hand, low-molecular-weight photoconductive compounds can readily yield photosensitive materials having excellent mechanical properties by selecting a suitable binder polymer, but cannot be said to have sufficiently high sensitivity. For example, the diarylalkane derivatives described in U.S. Pat. No. 3,820,989 have no appreciable problem from the viewpoint of compatibility with binder polymers, but their stability to light is poor. Accordingly, these compound have the disadvantage that, when they are used in the photosensitive layer of a electrophotographic sensitive material which is repeatedly charged and exposed to light, the sensitivity of this material is gradually reduced during repeated use. Moreover, the stilbene compounds described in Japanese Patent Laid-Open No. 65440/'83 have relatively high charge-holding capacity and sensitivity, but fail to exhibit satisfactory stability during repeated use.
The monostyryltriphenylamine compounds [for example, of formula (101)] described in Japanese Patent Publication No. 019867/'88, the distyryltriphenylamine compounds [for example, of formula (102)] described in Japanese Patent Publication No. 042661/'93 and Japanese Patent Laid-Open No. 120346/'87, and the tristyryltriphenylamine compounds [for example, of formula (103)] described in Japanese Patent Publication No. 093124/'94 and Japanese Patent Laid-Open No. 163361/'88 have high charge-holding capacity and sensitivity, and exhibit good stability during repeated use. However, they fail to give sufficient charge mobility, and are hence unsatisfactory as charge transport materials for use in high-speed electrophotographic sensitive materials. In these compounds as represented by formulae (101) to (103), only one triphenylamine structure [of formula (201)] serving as a hopping site for charges is contained in the molecule. Consequently, they fail to give sufficient charge mobility and, therefore, cannot exhibit sufficient sensitivity. Although various substituents may be introduced into these compounds, it has not been described therein to introduce a plurality of triphenylamine structures. Moreover, an electrophotographic sensitive material using a mixture of two or more of the aforesaid triphenylamine compounds is described in Japanese Patent Publication No. 013741/'95, but each compound contains only one triphenylamine structure. Thus, no compound containing a plurality of triphenylamine structures is described therein. ##STR3##
The triphenylamine compounds [of the following formula (2)] described in Japanese Patent Application No. 002844/'97 can satisfy the above-described requirements. ##STR4## wherein Ar.sub.1 to Ar.sub.6 each represent an unsubstituted phenyl radical or a substituted phenyl radical having one or more substituents on the phenyl nucleus in which the substituents may be the same or different and each comprise an alkyl group of 1 to 4 carbon atoms, an alkoxy group of 1 to 4 carbon atoms, an amino group, an alkylamino group of 1 to 4 carbon atoms, a dialkcylamino group of 1 to 4 carbon atoms, an alkylthio group of 1 to 4 carbon atoms, a halogen atom, or a halogenoalkyl group of 1 to 4 carbon atoms, and R.sub.1 to R.sub.6 may be the same or different and each represent a hydrogen atom or a methyl group.
The triphenylamine compounds of formula (2) can be prepared by the Wittig reaction for condensing a tricarbonyl compound such as 4,4',4"-triformyltriphenylamine or 4,4',4"-triacetyltriphenylamine with a phosphonic ester in the presence of a base. However, difficulty is frequently encountered in introducing a plurality of carbonyl groups into a compound, so that it has been difficult to synthesize the desired triformyl compound efficiently. Moreover, phosphorous esters of amino compounds are generally unstable, so that it has been difficult to prepare the desired compound efficiently.